This invention relates generally to automatic test equipment, and more particularly to automatic test equipment, having an amplitude calibration feature, for accurately measuring signals produced by an electronic device under test.
Automatic test equipment, also known as a tester, is commonly used in the manufacture of semiconductor devices to determine whether the devices contain manufacturing defects. In this way, defective devices can be identified before being incorporated into an electronic product, thereby minimizing the chance that the electronic product will fail prematurely in the field.
Semiconductor devices that process both analog and digital signals must be tested with testers that are capable of producing and receiving both analog and digital signals. Such devices are commonly called mixed-signal devices, and the testers that test these devices are known as mixed-signal testers.
A tester generally includes computerized control circuitry (test computer), data acquisition circuitry (capture instruments), driver/receiver channels (channels), and tester pins. Each tester pin connects a channel to an electrical node of a device under test (DUT). Further, each channel is coupled to a capture instrument that collects data for subsequent analysis by the test computer.
A tester generally operates by executing test programs. A typical test program includes numerous test functions that direct the tester to perform various operations such as setting-up the tester, applying a test signal to a node of the DUT, measuring an output signal at another node of the DUT, moving data from the local memory of the capture instrument to the main memory of the test computer, analyzing the data, comparing analysis results to specified limits, and organizing the analysis results for subsequent printout or display.
FIG. 1A shows a block diagram of prior art mixed-signal tester 100. A test engineer typically develops a test program on computer work station 104, and then loads the test program into main memory 136 of test computer 102 using system bus 144.
During execution of the test program, test computer 102 sends control signals to driver/receiver channels 108, 110, 112, and 114, and capture instruments 116, 118, 120, and 122 through test computer interface 106. In a typical mode of operation, the control signals direct driver/receiver channel 108 to receive an output signal produced by DUT 132 through tester pin 124, and send the output signal to capture instrument 116 using line 150. The control signals then typically direct capture instrument 116 to filter the output signal using anti-aliasing filter 155, and sample the output signal using sample-and-hold 138. The control signals might also direct capture instrument 116 to convert the sampled data to digital form (if necessary) using a-to-d converter 140, and store the resulting digital data in local memory 142.
Additionally, the test program may direct test computer 102 to move the digital data from local memory 142 to main memory 136 so that processor 134 can analyze the data using digital signal processing (DSP) techniques. Accordingly, test computer interface 106 receives the digital data from local memory 142 through internal bus 148, and presents the digital data to main memory 136 using internal bus 146.
Although mixed-signal tester 100 may be used to analyze data derived from an output signal generated by DUT 132, we have recognized that the signal processing technique used by tester 100 yields testers which often do not have the desired performance. For example, before being sampled by sample-and-hold 138, the output signal produced by DUT 132 typically passes through either an attenuation stage (not shown) or an amplification stage (not shown) in driver/receiver channel 108.
However, both the attenuation stage and the amplification stage can introduce error into output signals produced by DUT 132. In particular, they can introduce signal amplitude error into the output signals, thereby causing processor 134 to produce analysis results that do not have the required level of accuracy.
Further, it is known in the field of signal analysis that a signal having a bandwidth equal to B Hz is uniquely represented by a group of samples taken every 1/2 B seconds. For this reason, the output signal produced by DUT 132 is first presented to anti-aliasing filter 155 in capture instrument 116. Anti-aliasing filter 155 is a low-pass filter stage designed to eliminate any signal components occurring at frequencies greater than B Hz. However, anti-aliasing filter 155 can also be a source of signal amplitude error. In particular, anti-aliasing filters are often insufficiently flat in the passband.
Finally, in order to obtain a group of samples taken every 1/2 B seconds, sample-and-hold 138 samples the output signal using a sampling rate that is greater than or equal to 2 B Hz. Sample-and-hold 138 then presents the sampled data to a-to-d converter 140. However, both sample-and-hold 138 and a-to-d converter 140 can also be sources of signal amplitude error.
FIG. 1B shows calibration circuit 156, described in U.S. patent application Ser. No. 08/347,633, filed Dec. 1, 1994, assigned to the same assignee as the present invention, the disclosure of which is expressly incorporated herein by reference. Calibration circuit 156, typically located in the channel circuitry of a tester, is used to correct all sources of amplitude error in a signal path to a device under test. Further, calibration circuit 156 is meant to be used in a mixed-signal tester that is capable of producing and receiving analog signals having frequencies within the radio frequency (RF) range.
In order to correct the error sources in a signal path to a device under test, switch 160 is actuated to connect terminal 158 to terminal 162, which is as close as possible to the end of the signal path. Next, an external reference standard (not shown) is connected to terminal 162, and calibration measurements are performed by the tester. The external reference standard includes a series of fixed reference standards.
Switch 160 is then actuated to connect terminal 158 to cal ref 164, which is an internal reference standard. Cal ref 164 also includes a series of fixed reference standards. Once again, the tester performs calibration measurements, and then compares them with the measurements made using the external reference standard. The difference between the calibration measurements made using the external and internal reference standards is proportional to the error introduced in the signal path. The test engineer then programs the tester to correct the error.
Although calibration circuit 156 may be used to correct signal amplitude error contributed by circuit elements in a signal path of a tester, we have recognized that the correction technique used by calibration circuit 156 also yields testers that may not have the desired performance. For example, correcting the error introduced in a signal path by adjusting each measurement by a predetermined amount substantially increases test time. Further, the calculated difference between measurements made with the external and internal reference standards may also include some error.
Accordingly, it would be desirable to have a tester that can automatically correct signal amplitude error, across an entire frequency spectrum of interest, without substantially increasing test time. It would also be desirable to have a tester that has an internal calibration source that is traceable to a known standard.